The invention relates to methods for operating a mixed-potential exhaust-gas probe and circuit arrangements for carrying out these methods.
Mixed-potential exhaust-gas probes are utilized, for example, as gas sensors to detect the hydrocarbon concentration of the internal combustion engine or as NOx probes for detecting the nitrogen oxide component in the exhaust gas of internal combustion engines.
These probes are with respect to their configuration similar to the xcex-probes and are presented, for example, in the text of Bosch entitled xe2x80x9cKraftfahrtechnisches Taschenbuchxe2x80x9d, 22nd edition, 1995, starting at page 490.
In known mixed-potential exhaust-gas probes, the signal is measured: as a voltage between two electrodes; via the short-circuit current between the electrodes; or, by tapping the voltage measurable between the electrodes and dropping across a resistor.
It is an object of the invention to provide a method for operating a mixed-potential exhaust-gas probe wherein a highest possible selectivity is made possible with respect to the individual components of the exhaust gas even in the presence of sometimes very large transverse sensitivities.
Furthermore, it is an object of the invention to provide circuit arrangements which make possible to carry out the methods with a technically simple configuration and the least number of components.
By applying a constant external voltage, the probe can be adjusted to some extent to individual exhaust-gas constituents which are to be detected. The external voltage is different from the thermodynamic equilibrium voltage.
The external constant voltage is determined previously and preferably experimentally.
In this case too, the exhaust-gas probe is adjusted to a certain extent to the detection of individual gas components of the exhaust gas.
The magnitude of the current, which is to be applied to the probe ceramic, is determined experimentally.
The sensitivity of the probe can be considerably increased by the voltage or the current which is distinguished from the thermodynamic equilibrium voltage and the thermodynamic equilibrium current, respectively.
A voltage-polarized current measurement (that is, a measurement of the current which drops on the electrodes of the mixed-potential exhaust-gas probe) at constant external voltage can be realized in a technically very simple manner with an inverting operational amplifier. A voltage divider is connected to the non-inverting input of the operational amplifier and one of the electrodes of the exhaust-gas probes is connected to the inverting input of the operational amplifier. A reference resistor is arranged in the feedback loop of the operational amplifier.
A current polarized voltage measurement is made possible in a technically simple realizable manner with a non-inverting operational amplifier. The voltage measurement is a measurement of the voltage, which adjusts between the electrodes, when applying a constant current to the probe ceramic. A voltage divider is arranged at the non-inverting input of the operational amplifier and a reference resistor is arranged at the inverting input thereof. The exhaust-gas sensor is arranged in the feedback loop of the operational amplifier.
In an advantageous embodiment, switching means are provided via which switching can take place between the two circuit arrangements.